This invention relates to a stabilized multilayer structure which is suitable for use either as a photovoltaic cell or as a photoelectrode when immersed in an electrolyte. More particularly, the invention relates to a structure which comprises a base semiconductor, an effective layer of insulator material on said semiconductor, and an effective layer of conducting material on said insulator material. More particularly, this invention relates to a structure which comprises a base layer of silicon, an effective layer of oxide on said silicon, and an effective layer of conducting material on said oxide layer.
Photoelectrochemical cells are capable of generating direct electrical energy as well as providing a means for storage of solar energy. The basic photoelectrochemical cell comprises a photoelectrode, a counterelectrode and one reduction-oxidation or redox couple in an electrolyte. The simplest photoelectrode in the basic cell comprises a semiconductor with the front face illuminated by solar radiation and in direct contact with the redox-electrolyte solution which contains the redox couple. The back face of the semiconductor is connected to an insulated wire, and a voltage is generated between the back face contact and the counterelectrode with electrons traveling in an external circuit formed by the wires between the two electrodes; and ions pass through the electrolyte between the two electrodes, completing the electrical circuit. The junction between the redox-electrolyte solution and semiconductor photoelectrode is a diode junction which acts much the same as a p-n junction in a solid state solar cell; however, since the junction between the electrolyte and semiconductor is a property of the interface, its formation does not require the precise diffusion of dopant material into the semiconductor which is usually important in a solid state device. The photoelectrochemical cell therefore has substantial differences from conventional solid state photovoltaic cells. These differences lead to important advantages over conventional solid state photovoltaic cells, such as the ability to use a broader range of materials for efficient cell operation, the ability to avoid the constraints of lattice parameter matching between adjacent material layers, which is necessary for nearly all solid state photovoltaic devices, and the ability to use small grain size semiconductor material without any substantive decrease in solar conversion efficiency.
All these advantages of the photoelectrochemical cell have the potential of leading to lower costs for production and storage of energy; but unfortunately, photoelectrochemical cells have some difficulties: (a) cell lifetimes are extremely short due to cell malfunctions caused by unwanted corrosion effects arising at the junction of the photoelectrode and electrolyte solution, and (b) voltages and currents are often less than would be expected due to undesirable effects which arise, typically from flow of large currents the reverse of the desired direction (dark currents) or from recombination currents which diminish cell output and efficiency.
A number of publications have disclosed various attempts to prevent corrosion of the semiconductor photoelectrode in a photoelectrochemical cell. One approach has been to utilize thin protective metal films, particularly gold and platinum, over the base semiconductor (see T. Skotheim, I. Lundstrom, and J. Prejza, J. Electrochem. Soc.: 128, 1625 (1981)); however, the films must be thin in order to permit light to pass through to the semiconductor, and it is difficult to produce uniform, impermeable, thin metal layers and corrosion still occurs. Even if the problem of corrosion protection were solved by a thin metal film, the voltage output, and consequently cell efficiency, is substantially reduced due to the ease of reverse dark current flow across the semiconductor-metal film junction.
A second approach to stabilize the photoelectrode concerns the use of an ultra-thin layer of a wide band gap oxide, typically TiO.sub.2 or SnO.sub.2, over the base semiconductor (see A. J. Nozik, Second International Conference on Photovoltaic Conversion and Storage of Solar Energy, Aug. 8, 1978, Cambridge, England). Films such as TiO.sub.2 are transparent but are also insulating in character and if deposited with thickness sufficient to protect against corrosion, the photogenerated charge carriers cannot penetrate the insulating layer and thus the insulator layer prevents operation of the cell. SnO.sub.2 layers are also transparent to light, are more corrosion resistant than TiO.sub.2, and can be made conductive by doping; however, SnO.sub.2 has virtually no electrocatalytic activity (the ability to enhance the kinetic exchange between electrons in the conducting layer and the redox reaction in the electrolyte solution). Electrocatalytic activity is quite important in driving the desired redox-couple reaction in the electrolyte solution. Therefore, unless an electrocatalytically active layer is deposited on the SnO.sub.2 layer, a photoelectrochemical cell, which uses SnO.sub.2 alone as a corrosion protective layer, has an extremely low cell output.
A third approach to prevent photoelectrode corrosion concerns coating of the base semiconductor with an organic conductor layer (see R. Noufi, O. Tench, and L. F. Warren, J. Electrochem. Soc., 127, 2310 (1980)); however, severe problems are encountered in aqueous electrolyte solutions with the organic layers showing poor adhesion and at best providing protection for only a few days. A fourth corrosion protection scheme concerns formation of derivatized layers over the base semiconductor which are covalently bonded with the surface layer of the base semiconductor, but photoelectrode stability is maintained for only several days (see J. M. Bolts, A. B. Bocarsky, N. C. Palazzotto, E. J. Walton, N. S. Louis, and N. S. Wrighton, J. Am. Chem. Soc., 101, 1378 (1979)).
Consequently, none of these references discloses a photoelectrode which is stable for any time period in excess of several days, and which produces a high cell output with good efficiency. Accordingly, there is a need for an improved corrosion resistant photoelectrode which has a long lifetime and shows improved photocell output and efficiency in the conversion of electromagnetic radiation to electrical power.
A photoelectrode consisting of a silicon base which carries a first layer of silicon dioxide and a second layer of indium tin oxide is disclosed by Rajeshwar et al. in Report SERI/CP-211-1434, Abstracts of Presentation at the Fourth Electrochemical Photovoltaic Cell Contractor's Review Meeting, (sponsored by the U.S. Department of Energy, Oct. 16 and 17, 1981, Denver, Colo.) pp. 135-139. Similarly, a photoelectrode configuration consisting of a silicon base which carries a first layer of silicon oxide which is about 15 Angstroms thick, and a second layer of indium tin oxide which is about 400 Angstroms thick, and a final layer of ruthenium oxide on platinum, is disclosed by Thompson et al., J. Electrochem. Soc., 129, 1934 (1982). However, neither of these references recognizes the critical nature of the thickness of the insulating layer on the silicon base. In particular, these references fail to suggest that the proper adjustment of the insulator thickness could serve to alleviate back tunneling events while allowing the flow of current in the desired direction.
Skotheim et al. in J. Electrochem. Soc., 129, 1737 (1982) disclose the preparation of photoanodes based on single crystal n-silicon carrying an 8-10 Angstroms non-stoichiometric oxide layer on which platinum and polypyrrole films were deposited. However, this reference fails to either teach or suggest that the presence of an insulating layer on the semiconductor and the proper adjustment of the thickness of this layer is critical. In particular, this reference also fails to suggest that the proper adjustment of the insulator thickness could serve to alleviate back tunneling events while allowing the flow of current in the desired direction.
The general object of this invention is to provide an improved efficiency, high output, corrosion resistant photoelectrode in a photoelectrochemical cell.
A more specific object of this invention is to provide an improved efficiency, high output, corrosion resistant, silicon based photoelectrode in a photoelectrochemical cell with effective insulator and conducting layers on the base semiconductor.
A further object of this invention is to provide a multilayer structure which is suitable for use either as a photovoltaic cell in air or as a photoelectrode when immersed in a suitable electrolyte.
Other objects of this invention will be apparent to persons skilled in the art from the following appended claims.